The invention relates to a hybrid vehicle powertrain having an engine and a hydraulic drive. More particularly, the invention pertains to switching a pump/motor between pumping operation and motoring operation.
Hydraulic Power Assist (HPA) is a type of hydraulic hybrid vehicle, in which energy from regenerative braking or from an engine is stored in a hydro-pneumatic accumulator, and the conversion between mechanical power and hydraulic power is achieved through high pressure pump/motor having a variable volumetric displacement. In an HPA system, using stored energy from regenerative braking to help accelerate the vehicle reduces the burden on the engine and reduces fuel use.
Because of the high power density available with such hydraulic systems, it is possible to recover efficiently a significant portion of braking energy with an HPA system comprised of a single pump/motor and storage accumulators. With a 7000 lb. vehicle and a pump/motor whose maximum displacement is 150 cc., an HPA system can recover 72 percent of the available braking energy in the Environmental Protection Agency (EPA) city cycle. The pump/motor operates for long periods at higher displacements and with a relatively high cycle average efficiency of 88 percent. With a return of 56 percent of the braking energy to the drive wheels (72 percent recovered in braking, and 88 percent transfer efficiency in both pumping and motoring), it is possible to recover 56 percent of the vehicle kinetic energy (or 75 percent of the velocity) while accelerating, neglecting road load friction. In the EPA city cycle it was possible to fill the hydraulic system when braking from 30 mph and then moderately accelerate again to about 22 mph using only stored energy from the HPA system.
A hydraulic or pneumatic pump/motor operates in a pumping mode and a motoring mode. When changing operating modes in a hybrid hydraulic vehicle between motoring and pumping, the inlet and outlet ports of the pump/motor must be switched between connections to high pressure and low pressure sources by changing the state of several valves in a hydraulic system. This switching creates a sudden release of energy, which can place a large shock loading on the system. The control method of this invention minimizes the large shock associated with this high speed pressure switching.
In the pumping mode, hydraulic fluid is moved from a low-pressure reservoir to a high-pressure accumulator. The pump outlet pressure rises as the pump rotates and very quickly opens a check valve to begin forcing fluid into the accumulator. In the motoring mode, high pressure fluid leaving the accumulator drives the pump/motor in rotation and returns to the reservoir.
While pumping or motoring, displacement of the pump/motor can be independently controlled to vary the volume of fluid moved during each revolution of the pump/motor rotor between its inlet and outlet ports. When switching from pumping to motoring, it is necessary to connect the accumulator to either the inlet port or the outlet port of the pump/motor, and to connect the reservoir to the other of the two ports.
High pressure solenoid valves accomplish this switching, which must be done carefully to prevent unsafe or unpleasant conditions for the vehicle occupants. For example, when switching from pumping to motoring, a valve opens to allow high pressure fluid to flow to the inlet port of the pump/motor. There is an immediate rise in pressure in the hydraulic line causing a noisy shock wave to propagate toward the pump/motor that may vibrate the components. In addition, if the pump/motor has a positive, non-zero displacement, the wheels of the vehicle will be driven by torque transmitted from the pump/motor causing the vehicle to move.